A gas turbine aircraft engine comprises three primary components: the compressor, combustor and turbine. Air enters the gas turbine engine at the engine inlet and flows from there into the compressor. Compressed air flows to the combustor where it is mixed with injected fuel and the fuel-air mixture is ignited. The hot combustion gases flow through the turbine. The turbine extracts energy from the hot gases, converting it to power to drive the compressor and any mechanical load connected to the drive.
Both the compressor and the turbine consist of a plurality of stages. Each stage is comprised of a rotating multi-bladed rotor and a nonrotating multi-vane stator. Each vane and blade are of airfoil section.
The present invention is a mechanism incorporated in each of the blades of one or more rotor stages. The blades of the rotor are circumferentially distributed on a disk for rotation therewith about the disk axis. As shown in FIG. 1A, a conventional rotor blade 10 has a root or dovetail portion 16 (shown in its as cast condition before machining) which is slidably received in a complementarily configured recess (not shown) provided in the rotor disk, a platform portion 14 located outside the rotor disk and an airfoil portion 12 extending radially outwardly from the platform.
The platforms collectively define a radially outwardly facing wall of an annular gas flow passageway through the engine. The airfoils of the rotor blades extend radially into the passageway to interact aerodynamically with the gas flow therethrough.
These airfoils are subject to fatigue due to vibrations even though the angular speeds are low, for example, 4,000 rpm. It is necessary to damp such vibrations to reduce the fatigue on the blades, particularly at or near resonant frequencies.
Various types of blade dampers are known. For example, in a shroud-type damper, the distal ends of adjacent airfoils are physically connected to one another. This design has the disadvantage that the damper increases the mass of the rotor at the greatest radial distance and may interfere with gas flow through in the passageway.
Another known type consists of so-called under-platform dampers which generally have a movable member positioned between the rotor disk and the underside of the platform of one or more turbine blades. Upon rotation of the turbine, the member is displaced radially outwardly by centrifugal forces to engage the undersides of adjacent blades and thereby perform perform a damping function.
In yet another type of damper, a member is positioned between the platforms of adjacent blades. In accordance with the teaching of U.S. Pat. No. 2,912,223 to Hull, a spring-like member arranged between the platforms of adjacent blades is used to damp vibrations of the blades and seal the gap between adjacent platforms. U.S. Pat. No. 4,497,611 to Keller teaches an axial flow turbine wherein a wedge-shaped member is axially displaced by an axial pressure difference, whereby turbine blade vibration is damped.
In accordance with the teaching of U.S. Pat. No. 4,872,812 to Hendley et al., vibration damping and gap sealing are provided by elongated inserts of triangular cross section loosely received within pockets at the spacing gaps between between opposing convex and concave airfoil side edges of adjacent blade platforms. The inserts and pockets are relatively configured to provide self-orientation of the inserts into gap sealing and vibration damping engagement with the platform edges when acted upon by centrifugal forces due to rotor rotation.
An improved vibration damper for a turbine rotor blade was disclosed in U.S. Pat. No. 4,936,749 to Arrao et al. A U-shaped wire-form damping member has its legs respectively slidably received in a pair of inclined recesses which extend into the platform portion of the blade and toward the root portion. When the rotor disk is rotated at a sufficiently great angular speed, the damping members move outwardly due to the centrifugal force acting thereon to engage the opposing surface of the adjacent blade platform. When so engaged, the members damp vibrations of such blades and seal the space between the opposing platform surfaces.